This invention pertains to a three dimensional eye tracker, and more particularly pertains to an eye tracker for continuously measuring the point in three dimensional space on which the eye is fixated. That is, the eye tracker continuously tracks orientation of the optic axis of the eye and movement of the eye along the axis. This is accomplished by monitoring Purkinje images formed by reflecting surfaces within the eye.
There are many areas of both research and possibilities for commercial application in which it is desirable to have available electrical signals proportional to the orientation of the optic axis of a human observer's eye. Research on the mechanics of eye movement, on many areas of visual perception and on visual search patterns would all be greatly aided by such a device. Commercial applications include the use of the eye to control other systems. For example, a computer operator might simply look at any one of a number of symbols or locations displayed in front of him and press a button, introducing the value corresponding to that location into the computer. Similarly, a servo system for controlling a gun or camera could automatically be aimed at whatever object an observer visually was fixating.
The prior art contains many methods for attempting to track eye movement. These methods, their advantages and disadvantages are dealt with at length in three United States patents assigned to Stanford Research Institute, now known as SRI International, the assignee of the present patent application, said three patents all having as one of their inventors, Hewitt D. Crane, an inventor of the present application, as follows: U.S. Pat. No. 3,712,716 issued Jan. 23, 1973 to Tom N. Cornsweet and Hewitt D. Crane for "Eye Tracker," U.S. Pat. No. 3,724,932 issued Apr. 3, 1973 to Tom N. Cornsweet and Hewitt D. Crane for "Eye Tracker and Method," and U.S. Pat. No. 3,804,496 issued Apr. 16, 1974 to Hewitt D. Crane and Tom N. Cornsweet for "Two Dimensional Eye Tracker and Method for Tracking an Eye." Reference should be had to these patents for a discussion of the early trackers. The information is not repeated here, but the subject matter of those patents is specifically incorporated herein by reference.
The information and characteristics of Purkinje images in the eye are also described in detail in the above referenced patents. Additionally, they are described in "Accurate Two-Dimensional Eye Tracker Using First and Fourth Purkinje Images" by T. N. Cornsweet and H. D. Crane, JOSA, v. 63, n. 8 (Aug. 1973), pp. 921-928, and "Accurate Three-Dimensional Eyetracker" by H. D. Crane and C. M. Steele, App. Optics, v. 17, n. 5 (Mar. 1, 1978), pp. 691-705. In view of the full treatment of the subject of Purkinje images in these patents and articles, the information is not repeated here. The two articles, however, are also specifically incorporated herein by reference. Note also that the JOSA article describes a two dimensional Purkinje eye tracker and the Applied Optics article describes the present three dimensional Purkinje image eye tracker.
The two dimensional eye trackers disclosed and claimed in the above referenced patents have been highly successful and have solved many of the problems. For example, they do provide eye trackers which require no attachments to the eye, which produce tracking signals much more accurate than 1/2.degree., and which are able to distinguish between translation artifacts and rotation of the eye. The eye trackers continuously monitor the orientation of the optic axis of the eye using first and fourth Purkinje images. As their designation indicates, however, the instruments only measure the direction of the visual axis and cannot accommodate axial variation in eye position.